This application is related to copending application Ser. No. 10/628,188 filed concurrently herewith for, “Reducing Nitrogen Content in Silicon Carbide Crystals by Sublimation Growth in a Hydrogen Containing Ambient.”
The present invention relates to the growth of ultra high purity semi-insulating silicon carbide crystals in a hydrogen ambient that yields a low nitrogen concentration in the crystal to enhance the semi-insulating qualities.
Silicon carbide (SiC) has a combination of electrical and physical properties that make it an attractive semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 electron-volt (eV) bandgap (6H), a 4 Megavolt per centimeter (MV/cm) electric field breakdown, a 4.9 W/cmK thermal conductivity, and a 2×107 centimeter per second (cm/s) electron drift velocity. Silicon carbide is also particularly useful in its ability to be made conductive by doping or semi-insulating by various processing techniques. These qualities make silicon carbide a material of choice for a vast array of electronic applications.
The production of integrated circuits for many applications, such as RF devices, requires a semi-insulating substrate on which electronic devices can be built and connected to one another. Historically, sapphire was used as substrate material for microwave devices because of its high resistance to current flow. Sapphire has the disadvantage, however, of limiting the types of semiconductor layers that may be fabricated on the substrate with appropriate crystal lattice matching for proper device operation.
As recognized by those familiar with semiconductor electronics, certain devices often require high resistivity (“semi-insulating”) substrates to reduce RF coupling or for other functional purposes such as device isolation because conductive substrates tend to cause significant problems at higher frequencies. As used herein, the terms “high resistivity” and “semi-insulating” can be considered synonymous for most purposes. In general, both terms describe a semiconductor material having a resistivity greater than about 1500 ohm-centimeters (ohm-cm).
In general, semi-insulating silicon carbide devices should have a substrate resistivity of at least 1500 ohm-centimeters (ohm-cm) in order to achieve RF passive behavior. Furthermore, resistivities of 5000 ohm-cm or better are needed to minimize device transmission line losses to an acceptable level of 0.1 dB/cm or less. For device isolation and to minimize back-gating effects, the resistivity of semi-insulating silicon carbide should approach a range of 50,000 ohm-cm or higher.
Research in the field shows that the semi-insulating behavior of a silicon carbide substrate is the result of energy levels deep within the band gap of the silicon carbide; i.e., farther from both the valence band and the conduction band than the energy levels created by p-type and n-type dopants. These “deep” energy levels are believed to consist of states lying at least 300 meV away from the conduction or valence band edges, e.g., U.S. Pat. No. 5,611,955 which is representative of standard prior research in this art.
Various devices fabricated in silicon carbide require different degrees of conductivity to provide accurate electrical responses, such as current switching, signal amplification, power transfer, etc. In fact, the desired electrical response of a silicon carbide crystal can range from a highly conductive crystal to a highly resistive (semi-insulating) crystal. Silicon carbide grown by most techniques is generally too conductive for semi-insulating purposes, however. In particular, the nominal or unintentional nitrogen concentration in silicon carbide tends to be high enough in sublimation grown crystals (≧1–2×1017/cm3) to provide sufficient conductivity to prevent silicon carbide from being used in devices that require a semi-insulating substrate, such as microwave devices.
A recurring issue in fabricating silicon carbide crystals for electronic devices, therefore, is the control of elemental impurities such as nitrogen within the crystal. Nitrogen content, for example, affects the color of a silicon carbide crystal. This color change can have deleterious consequences for the usefulness of a crystal in certain applications requiring luminescence, such as light emitting diodes and gemstone fabrication. The nitrogen in a crystal may also yield electrical conductivity that must be controlled for silicon carbide to have appropriate properties in diverse electronic applications. The invention herein includes a means for achieving a semi-insulating silicon carbide crystal one step of which comprises reducing the nitrogen content, and therefore the inherent conductivity of a crystal with an improved method of sublimation growth in a hydrogen ambient atmosphere.
Researchers, therefore, persistently struggle with the issue of controlling, and particularly reducing, the amount of nitrogen that is transferred from the atmosphere of a sublimation growth chamber into a growing silicon carbide crystal. Commonly assigned U.S. Pat. No. 5,718,760 to Carter et al., for example, discloses a method of reducing the nitrogen concentration in the ambient atmosphere of a silicon carbide sublimation system. The Carter '760 patent reduces the nitrogen by back filling the growth chamber with an inert gas such as argon and then evacuating the growth chamber to a very low pressure.
Another technique for decreasing the ambient nitrogen in a crystal growth system is the minimization of nitrogen content in the equipment itself. Commonly assigned U.S. Pat. No. 5,119,540 issued to Kong et al., discloses that most, if not all, of the undesired nitrogen in a crystal growth system is a result of nitrogen gas that escapes from the equipment itself. For example, nitrogen trapped in graphite equipment may leak into the ambient atmosphere because the equipment cracks or develops pin holes through which nitrogen escapes at very high temperatures. The Kong '540 patent prevents incorporation of nitrogen into subject silicon carbide crystals by utilizing fabrication equipment made of materials with low nitrogen concentration. The Kong '540 patent, therefore, teaches that extremely pure equipment components that are free of high nitrogen content result in silicon carbide crystals that are less contaminated with undesirable levels of nitrogen. Kong '540 shows nitrogen minimization in a chemical vapor deposition system but is equally pertinent in the sublimation systems discussed herein.
In addition to reducing the concentration of nitrogen, researchers also reduce the effects of unavoidable nitrogen content within a silicon carbide crystal. For example, the Carter '760 patent acknowledges that the background nitrogen in the sublimation chamber can lead to undesirable crystal color. The '760 patent, therefore, discloses a method of compensating the nitrogen content with a corresponding p-type dopant to minimize or eliminate the undesirable effects of the nitrogen. The p-type dopant and the nitrogen compensate one another and prevent undesirable color centers in the preferably colorless silicon carbide crystal of the Carter '760 invention.
The nitrogen compensation technique has also been used to prevent unintentional nitrogen doping from dominating the conductivity of silicon carbide crystals. Commonly assigned U.S. Pat. No. 6,218,680, also issued to Carter et al., discloses a further method of compensating the nitrogen content of a silicon carbide crystal grown by sublimation. Carter points out that boron may be used to compensate the inherent nitrogen. Carter '680 also utilizes the temperature gradient in the disclosed sublimation process to create point defects in a silicon carbide crystal. The Carter '680 technique pairs an undesirable nitrogen concentration in the silicon carbide crystal with a corresponding acceptor dopant, such as boron. Carter '680 then pairs any excess dopants with temperature induced point defects to yield a desired semi-insulating crystal.
Other research also concedes that unintentional nitrogen incorporation occurs in silicon carbide crystals grown by sublimation. This research tends to focus on means for minimizing the effects of the undesirable nitrogen concentration instead of preventing the nitrogen incorporation from the outset. U.S. Pat. No. 5,611,955, issued to Barrett et al. is illustrative of this point. Barrett '955 shows a means of introducing elements such as vanadium into the semiconductor material that create deep energy states within the forbidden energy gap. The Barrett '955 method accounts for nitrogen content in a silicon carbide crystal by trapping the nitrogen and hindering electron mobility from the nitrogen. Barrett, therefore, achieves a semi-insulating silicon carbide substrate by adjusting the effects of the nitrogen instead of preventing its presence in the crystal.
The techniques set forth in the two Carter patents, which have a common assignee as the invention described and claimed herein, are useful for their respective purposes to minimize the effects of nitrogen incorporation in a silicon carbide crystal. The Barrett '955 patent requires further elemental doping and can give rise to unpredictable electrical responses in a subject silicon carbide crystal.
A need continues to exist, therefore, for a method of gaining extensive control over the incorporation of nitrogen into a silicon carbide crystal at the point of initial sublimation. By controlling the nitrogen content from the initial growth of the crystal, compensation techniques and the associated process steps may be minimized. Controlling the nitrogen incorporation also allows development of more diverse types of crystals, including crystals with varying degrees of nitrogen content for specialized purposes.
The method described and claimed herein provides a technique for fabricating semi-insulating silicon carbide crystals with a more predictable resistivity than methods of the prior art. Gaining control over the amount of nitrogen incorporated into a silicon carbide crystal grown by sublimation is a critical improvement in sublimation processes and yields a more reliable, higher quality semi-insulating silicon carbide crystal product.